The present invention relates to a method for improving field homogeneity during magnetic resonance imaging when the homogeneity has been degraded by local susceptibility. More specifically, the invention relates to a method of placing a material with a magnetic susceptibility similar to body tissue in proximity to a part of a patient's body being imaged with a magnetic resonance imaging system and then imaging that body part, whereby field homogeneity is improved such that a homogeneity sensitive protocol provides an improved image of the body part.
Magnetic resonance imaging, or MRI, is a method by which the location, size, and conformation of organs and other structures of the body may be determined. In the typical MRI system, a magnetic field is established across a body to align the spin axes of the nuclei of a particular chemical element, usually hydrogen, with the direction of the magnetic field. The aligned, spinning nuclei execute precessional motions around the aligning direction of the magnetic field. For the aligned, spinning nuclei, the frequency at which they precess around the direction of the magnetic field is a function of the particular nucleus which is involved and the magnetic field strength. The selectivity of this precessional frequency with respect to the strength of the applied magnetic field is very short and this precessional frequency is considered a resonant frequency.
In an ordinary MRI system, after the nuclei have been aligned or polarized, a burst of radio frequency energy at the resonant frequency is radiated at the target body to produce a coherent deflection of the spin alignment of the selected nuclei. When the deflecting radio energy is terminated, the deflected or disturbed spin axes are reoriented or realigned, and in this process radiate a characteristic radio frequency signal which can be detected by an external coil and then discriminated in the MRI system to establish image contrast between different types of tissues in the body. MRI systems have a variety of different excitation and discrimination modes available, such as free induction decay (“FID”), spin echo, and continuous wave, as are known in the art.
Two parameters are used to measure the response of the magnetized sample to a disturbance of its magnetic environment. One is T1 or longitudinal relaxation time, the time it takes the sample to become magnetized or polarized after being placed in an external magnetic field; the other is T2, the spin relaxation time, a measure of the time the sample holds a temporary transverse magnetization which is perpendicular to the external magnetic filed. Images based on proton density can be modified by these two additional parameters to enhance differences between tissues.
Hydrogen is usually selected as the basis for MRI scanning because of its prominent magnetic qualities. Hydrogen, having a single proton nucleus, is easily polarized. Further, hydrogen is abundant in water, a major component of the human body. Tissues that have a high content of water, and thus hydrogen and hydrogen protons, are deemed “protonated” and provide strong images during MRI. One disadvantage to the abundance of hydrogen in the human body, however, is that signal from tissues of little or no interest may obscure signal from more diagnostically relevant adjacent tissues.
The images formed in magnetic resonance imaging are really a converted visual display of the otherwise invisible radio waves emitted by protons (when scanning for hydrogen atoms) that are detected by the MRI pick-up coil. When scanning for hydrogen atoms, tissue areas which have no hydrogen atoms emit no radio waves, and thus the MR image of this tissue is displayed with no intensity, and appears black. Tissues that have a high hydrogen content, on the other hand, may emit a large amount of radio waves depending on the scanning criteria. Such signals are converted into a correspondingly bright visual display image. Normally, grey scale assignment, based upon the relative energy or signal intensities received from the tissues, is utilized in order that the user may more easily distinguish the various tissues and organs imaged. On these grey scale images, low or no signal is designated as black, and very high signals are assigned a lighter shade of grey or even white.
Occasionally, tissues that are in abundance and create a bright signal may overwhelm the signal emanating from less abundant and differently hydrogenated species or tissue. This may visually mask the latter tissue and obscure a disease process or anatomy. This decreases the sensitivity of MRI for certain disease processes and creates a problem for the diagnostician.
Various methods have been used in order to try and separate the signals coming from the various tissues of the body and thereby produce more distinct images. One such method involves nullifying the signal received from a certain tissue. This is done by utilizing spin echo and gradient echo presaturation pulse sequences based upon information about subtle differences in the precessional frequency of hydrogen atoms as they associate with fatty versus non-fatty tissues. For example, in order to improve the conspicuity of non-fatty tissues that lie in a background of a fatty tissue, the entire tissue is first subjected to a chemically specific saturation radio pulse. This preparatory pulse essentially affects the hydrogen atoms associated with the fat molecules. These pretreated hydrogen atoms have, in a sense, been briefly deactivated and are not able to emit a useful signal when the actual imaging portion of the pulse sequence commences. The MR image is then created with little or no contribution from the fatty tissue. The resultant image will show the non-fatty tissue against a dark background. This process is called chemically selective presaturation of fat or fat saturation.
The precessional frequency difference between fat and water is known as chemical shift and has a value of 3.5 parts per million (ppm) of the field strength. At a 1.5 T field strength, the 3.5 ppm chemical shift causes fat to precess 220 Hz slower than water. This fat saturation process is unreliable if the inhomogeneity obscures the precessional difference between fat and water. A water tissue in an inhomogeneous field 220 Hz lower than the main field strength is difficult to distinguish from a fat tissue in the main field strength which is precessing 220 Hz slower. Because the precession frequency differences between the fatty and non-fatty tissues are very minute, the homogeneity must be very precise or non-fatty tissues are inadvertently variably saturated themselves. This problem is further compounded by the fact that the local magnetic environment of tissues tends to change based upon their position relative to the coil; position in the magnetic bore; and position with respect to organs or tissues with different magnetic susceptibilities (e.g. tissue next to bone or tissue next to air). Not only is the immediate magnetic environment important, but also the actual geometry of the organ or body part plays a major role in determining the fatty tissue's likelihood of being nullified with the fat saturation technique.
Further, interpretive problems can arise in several ways. First, if the fat is not saturated effectively, then pathology can be obscured. Second, if the fat is saturated in only portions of the body part being imaged, then the areas not saturated may be misinterpreted as pathologic tissue. Third, drastic alteration in geometry and magnetic susceptibility which naturally occur in the neck, shoulders and ankle, for example, can lead to inappropriate saturation of non-fatty tissues which are the subject of the examination.
One method occasionally used to improve magnetic field homogeneity by addressing the above stated limitation of this technique involves placing water bags around the body part being scanned. This technique is useful in that there is improvement in the local homogeneity and hence in the quality and reliability of the fat saturation technique. This is based on effectively changing the geometry of the body part. The bags must also conform closely to the body so as to eliminate or minimize any air gap between the bag and the body, referred to as the tissue-air interface.
Water, however, is highly protonated and creates a correspondingly bright signal surrounding the fat site. The bright background is a serious disadvantage for this procedure because it is distracting and counteracts the improved visualization produced by using water-filled bags with fat saturation sequences. It also causes a pseudo increase in the intensity dynamic range of the display device, leading to poor contrast in the pathologically relevant part of the image.
It has also been shown that fluorocarbons which have a magnetic susceptibility similar to that of human tissue, improve fat saturation when they are placed next to tissue being scanned using MRI. In particular, fluorocarbon materials containing little or no hydrogen, when placed around a body part being scanned, effectively eliminate the skin-air interface, eliminate magnetic susceptibility differences, dramatically improve the fat saturation efficacy, and add no signal of their own to the final image.
Unfortunately, fluorocarbons that are acceptable for use in the MRI imaging field are generally very expensive. Some forms of fluorocarbons have very low viscosity and will leak out of any enclosure rapidly and cause a slippery mess. Slippery fluid on the exterior of the container also prevents adhesion, thus making the enclosures difficult to seal initially and difficult to repair in the event of a puncture. Also, material options for containing fluorocarbons are limited because, in addition to proton imaging, many high field scanners also do multinuclear spectroscopy. Multinuclear spectroscopy is a procedure in which data is collected from nuclei other than protons (Hydrogen). One of the nuclei is Fluorine. Therefore, the presence of perflourocarbon would be very disruptive to Fluorine spectroscopy.